KR101449357B1 - Optical encoder - Google Patents

Abstract

The optical encoder includes a transparent portion having a periodic optical pattern and having a relatively angular displacement, a transparent portion for irradiating light toward the optical scale, a light receiving portion for receiving light from the optical scale, And an arithmetic section for calculating the absolute angle of rotation? Of the optical scale on the basis of the angle? The optical pattern includes a plurality of light shielding portions and a plurality of transmission portions alternately arranged, and when the pitch of the nth light shielding portion along the predetermined circumferential direction is Pn and the width of the nth light shielding portion is Wn, The transmittance T (n) corresponding to the angle? N of the light shielding portion and the pitch Pn of the light shielding portion satisfy a predetermined formula and the width Wn of the light shielding portion of the optical pattern changes as a function of the pitch Pn of the light shielding portion. the pitch Pn and the width Wn of the nth light-shielding portion 6a are preferably in inverse proportion.
With such a configuration, it is possible to achieve an optical encoder having a high accuracy and a high resolution as compared with the conventional one.

Description

[0001] OPTICAL ENCODER [0002]

The present invention relates to an optical encoder capable of detecting an absolute rotation angle of an optical scale.

Generally, a rotary encoder for detecting a rotation angle of a measurement object includes an optical scale having an optical pattern of light and dark, a detection element for detecting an optical pattern on an optical scale, and a calculation device disposed at the rear end of the detection element , The rotation angle of the optical scale connected to the rotation axis of the motor or the like is detected by the computing device.

As rotary encoders of this type, there are an incremental system in which a pulse signal outputted from a detection element is integrated by a computing device to detect a rotation angle, and an absolute angle of an optical scale Is known. In the incremental system, since the rotation angle is detected by the increment from the origin position, the origin return operation is required at the time of turning on the power. On the other hand, the absolute system does not need to integrate the pulse signals, so that the origin return operation is not required at the time of turning on the power supply, and it is possible to quickly recover from the emergency stop or power failure.

As a technique for detecting an absolute angle or an absolute position, a method of modulating an optical pattern of an optical scale is known. For example, the linear scale measuring apparatus of Patent Document 1 has an optical scale comprising a transparent portion and an opaque portion, a transparent portion for irradiating light to the optical scale, and a light receiving portion for receiving light from the optical scale, The line width of the non-transparent portion is sequentially changed from a thin line to a thick line, and the amount of transmitted light detected at the light receiving portion is changed in a sinusoidal manner.

Further, the position detecting device of Patent Document 2 has an optical scale comprising a light shielding portion and a transmission portion, a light projecting portion for irradiating light to the optical scale, and a light receiving portion for receiving light from the optical scale, The length of the light portion is modulated so that the absolute amount of light passing through the optical scale is monotonously reduced or monotonically increased.

JP-A-61-182522 (Fig. 6) Japanese Patent Laid-Open No. 2007-248359 (Figs. 1 and 2) Japanese Patent Application Laid-Open No. 2003-177036 Japanese Patent Application Laid-Open No. 2003-75200 Japanese Patent Application Laid-Open No. 2005-164533

In Patent Document 1, a sinusoidal wave output can be obtained by modulating an optical pattern on an optical scale. Further, by providing a light receiving element at a position shifted by a quarter cycle, an output of the sinusoidal wave is obtained. it is possible to detect the absolute angle by arctangent calculation.

Incidentally, when the bottom portion of the sine wave output rises due to the positional deviation or the like of the optical system, an offset error a occurs. In this case, if the offset of the sinusoidal output is a and the amplitude is b, the result of the inverse operation is as shown in the following equation, and an error epsilon is generated.

That is, since the angle error? Is determined by the magnitude of the offset error a with respect to the amplitude b, the value of a / b becomes smaller by increasing the amplitude b, and the influence of the offset error a can be reduced. In the case of the optical scale according to Patent Document 1, in order to increase the amplitude and increase the difference between the maximum light amount and the minimum light amount, it is necessary to minimize the width of the transparent portion at the portion corresponding to the bottom portion of the sinusoidal wave, The width of the opaque portion at the portion corresponding to the peak portion of the sinusoidal wave must be minimized.

Further, in Patent Document 2, it is possible to obtain a triangular wave or a sinusoidal wave if it is a linear encoder. However, in order to realize a sinusoidal wave output by a rotary encoder, an optical pattern of an arc shape is required. In addition, the design method of the optical scale changes in the linear encoder and the rotary encoder, and the design becomes troublesome.

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical encoder which is high-precision and high-resolution compared to the prior art.

In order to achieve the above object, an optical encoder according to the present invention comprises:

An optical scale having a periodic optical pattern, a relatively angular displacement,

A transparent portion for irradiating light toward the optical scale,

A light receiving unit for receiving light from the optical scale,

An operation unit for calculating an absolute rotation angle? Of the optical scale based on a signal from the light receiving unit,

The optical pattern includes a plurality of light-shielding portions and a plurality of light-transmitting portions alternately arranged,

When the pitch of the nth light-shielding portion along the circumferential direction is Pn and the width of the nth light-shielding portion is Wn, the transmittance T (n) corresponding to the angle n of the nth light- Satisfy the following formulas (A1) and (A2)
The width Wn of the light-shielding portion of the optical pattern is characterized by being expressed by the following expression (A3) using the pitch Pn of the light-shielding portion, the constant A, and the coefficient m (m is a real number larger than 0).

According to the present invention, both the pitch Pn of the light-shielding portion and the width Wn of the light-shielding portion change along the circumferential direction of the optical scale, so that it becomes possible to obtain an arbitrary transmittance according to the rotation of the optical scale. Therefore, the difference between the maximum amount of light and the minimum amount of light can be made larger as compared with the modulation of only the pitch or the modulation of only the width. As a result, the absolute rotation angle? Of the optical scale can be measured with high accuracy and high resolution.

1 is a perspective view showing a configuration of Embodiment 1 of the present invention,
2 is a sectional view showing an example of an optical scale,
3 is a cross-sectional view showing another example of the optical scale,
4 is a graph showing the output signal of the light receiving unit,
5 is a graph showing the result of subtracting the offset from the output signal of the light receiving unit,
6 is a graph showing an offset error due to light quantity variation,
7 is an explanatory view showing the dimensions of the optical scale,
Fig. 8 is an explanatory diagram showing Embodiment 2 of the present invention. Fig.

(Example 1)

1 is a perspective view showing a configuration of a first embodiment of the present invention. The optical encoder 1 includes a transparent portion 2, an optical scale 3, a light receiving portion 4, an operation portion 5, and the like.

The transparent portion 2 is a light source for irradiating light toward the optical scale 3, and it is preferable to use an LED (light emitting diode) from the viewpoints of, for example, life span and cost. The LED may be a lens attached or a single chip for low cost. An optical system such as a lens or a mirror may be provided between the transparent portion 2 and the optical scale 3.

The optical scale 3 is supported so as to be able to be angularly displaced relative to the transparent portion 2 and the light receiving portion 4 and is provided with a plurality of shielding portions 6a and a plurality of transparent portions 6b alternately arranged along the circumferential direction (6). ≪ / RTI > The optical pattern 6 functions as a light intensity modulating means for modulating the light intensity irradiated by the light-transmitting portion 2.

In this embodiment, a transmission type encoder in which the optical scale 3 is placed between the transparent portion 2 and the light receiving portion 4 is exemplified, but on the other hand, the transparent portion 2 and the light receiving portion 4 ) May be disposed in the same manner. The structure of the optical scale 3 is not particularly limited as long as the periodic structure of the transmissive portion and the light shielding portion or the periodic structure of the reflective portion and the non-reflective portion is formed.

The optical scale 3 may be formed, for example, by depositing a metal such as chromium on a glass substrate and patterning the metal film by photolithography. As shown in Fig. 2, a transparent resin such as polycarbonate is used as a base material, and the transparent portion 6b is flat and the light-shielding portion 6a is formed as a projection having a V- It may be. In this case, if the angle of the protrusion is made to be equal to or larger than the critical angle of the light to be used, the light irradiated on the V-shaped protrusion is not transmitted by the total reflection, and functions as the light-shielding portion 6a. As shown in Fig. 3, the light-shielding portion 6a may be a groove having a V-shaped cross section and exhibits a similar shielding function. By adopting such an integral molded article, it becomes possible to manufacture the optical scale 3 at low cost.

In the optical scale 3, at least two tracks having a periodic optical pattern 6 are provided concentrically. For example, the optical pattern 6 provided on the first track T1 modulates the light intensity with a sine wave function of one cycle per rotation of the optical scale 3, while the optical pattern 6 provided on the second track T2 6 modulates the light intensity with a sinusoidal wave function of one cycle per rotation of the optical scale 3. That is, in the optical pattern 6 of each of the tracks T1 and T2, the light-shielding portion 6a and the transmissive portion 6b have the same distribution in the main direction, but are arranged so as to be out of phase with each other by 90 degrees.

The light receiving portion 4 is a light detecting means for receiving light from the optical scale 3, for example, transmitted light or reflected light, and outputting a signal proportional to the received light intensity, and for example, a light receiving element such as a PD (photodiode) is employed. Between the light receiving unit 4 and the optical scale 3, an optical system such as a lens or a mirror may be provided. The light receiving section 4 has two light receiving elements 41 and 42 corresponding to the respective tracks T1 and T2 of the optical scale 3. When the optical scale 3 makes one rotation, the light receiving element 41 outputs a signal S1 that changes in a sinusoidal wave, and the light receiving element 42 outputs a signal S2 that changes in a sine wave.

The calculation unit 5 is constituted by an A / D converter or a microprocessor and calculates an absolute rotation angle? Of the optical scale 3 based on the signals S1 and S2 from the light receiving unit 4. This calculation method will be described later in detail.

Next, the operation will be described. The optical scale 3 is connected to a rotating shaft of a rotating body such as a motor or a rotor and intensity-modulates the light from the transparent portion 2 in accordance with the rotation angle. The light modulated by the optical pattern 6 of each of the tracks T1 and T2 is detected by the light receiving elements 41 and 42 of the light receiving section 4, respectively.

When the optical scale 3 is rotated, a sinusoidal output S1 = (a + b x sin &thetas;) having an offset a and an amplitude b is obtained from the light receiving element 41 as shown in Fig. Loses. Similarly, the photodetector 42 obtains an excitation wave output S2 = (a + b x cos?) Having an offset a and an amplitude b.

The arithmetic unit 5 stores the previously measured offset value a as a correction value. 5, the calculation unit 5 subtracts the offset value a from the sine wave output S1 and the sine wave output S2 of the light receiving unit 4 to obtain a sinusoidal wave output S3 = b x sin &thetas; An output S4 = b x cos? Is obtained. Subsequently, the calculating unit 5 can calculate the absolute rotation angle? Of the optical scale 3 by executing the reciprocal calculation using the following equation (2).

Next, the angular error in the case where the light quantity fluctuation occurs will be described. Now, it is said that the light quantity fluctuation occurred by some influence. At this time, as shown in Fig. 6, the original output S1 is changed to the output S1? By adding the offset error?. In this state, even when the operation unit 5 performs the subtraction of the offset value a, the offset error? Remains, and if the inverse operation is performed as it is, an angular error? Is generated as shown in the following equation (3).

Therefore, as is apparent from the equation (3), it is necessary to increase the amplitude b in order to reduce the influence of the offset error?.

Next, a method for increasing the amplitude b will be described. Hereinafter, as shown in Fig. 7, a resin material is used as the base material of the optical scale 3, and a protrusion having a V-shaped cross section is formed as the light shielding portion 6a. However, Or the optical scale 3 having the periodic structure of the reflective portion and the non-reflective portion is not particularly limited.

In the optical scale 3 for outputting a sinusoidal wave of one rotation per rotation, the transmittance T (?) Of the optical scale 3 with respect to the angle is expressed by the following equations (4) and As shown in Fig. Here, [theta] n is the angle of the n-th light-shielding portion from the reference angle [theta] 0 along the predetermined circumferential direction.

If the pitch of the n-th light-shielding portion 6a is Pn and the width of the n-th shielding portion 6a is Wn, the transmittance T (n) corresponding to the angle n of the n-th light- (5).

The angle? N of the nth light-shielding portion can be defined by the following equation (6) using the pitch Pm of the mth light-shielding portion 6a.

Here, the pitch Pn and the width Wn of the nth light-shielding portion 6a are defined by the following equations (7) and (7a) by using the constant A.

When the pitch Pn of the light-shielding portion 6a is large, the width Wn of the light-shielding portion 6a is made small, and when the pitch Pn of the light-shielding portion 6a is small, The width Wn of the light portion 6a can be increased. As a result, the difference between the maximum light quantity and the minimum light quantity becomes larger, and the light intensity with a larger amplitude can be obtained in the light receiving section 4. [

The transmittance TL (?) At the peak position of the sinusoidal wave and the transmittance TL (?) At the bottom position of the sinusoidal wave are PH at the pitch of the light shielding portion 6a at the peak position of the sinusoidal wave and WH The pitch of the light-shielding portion 6a at the bottom position of the sine wave is denoted by PL, and the width of the light-shielding portion 6a is denoted by WL, it is expressed by the following equations (8) and (9).

Using the maximum transmittance TH and the minimum transmittance TL, the direct current component DC and the alternating current component AC of the sinusoidal wave are expressed by the following equations (10) and (11), respectively.

Using the equations (4) and (5), the pitch Pn of the light-shielding portion 6a is expressed by the following equation (12).

Substituting equation (7) into equation (12), equation (13) below is obtained.

For example, assuming that the width PH of the light-shielding portion 6a at the peak position of the sinusoidal wave is 1 degree (degree) and the pitch PH of the light-shielding portion 6a at the peak position of the sine wave is 10 degrees, = 10 and the transmittance at the bottom position of the sinusoidal wave is TL = 5%, the pitch Pn of the shielding portion 6a with respect to the n-th angle and the angle? N between the nth shielding portion 6a are Table 1).

Here, when n = 0 is defined as the origin, it is necessary to modify the angle of the last light-shielding portion 6a to match the origin. Therefore, an angle? N 'corresponding to the position of the nth light-shielding portion 6a after correction is corrected according to the following equation 14 using the angle? Of the last light-shielding portion 6a.

At this time, the pitch Pn of the light-shielding portion 6a with respect to the n-th angle and the angle? N 'between the n-th shielding portion 6a are as shown in the following Table 2.

Here, a case is assumed in which the width W of the light-shielding portion 6a is constant and modulation is performed only at the pitch Pn of the light-shielding portion 6a. The pitch PH of the light shielding portion 6a at the peak position of the sinusoidal wave is 10 degrees, the transmittance TH is 90%, the transmittance TL at the bottom position of the sine wave TL = 5%. The nth transmittance T (? n) can be expressed by the following equation (15) as shown in equation (5).

If W = 1 and TL = 5% are substituted into equation (15), PL = 1.05 degrees. Therefore, the interval between the light-shielding portions 6a at the bottom of the sinusoidal wave is 0.05 degrees. This corresponds to about 8.7 mu m when the radius of the optical scale 3 on the optical pattern 6 is 10 mm, for example.

On the other hand, substituting the equation (7) into the equation (9) yields the following equation (16).

When TL = 5% and A = 10 are substituted into the expressions (16) and (7), WL = 3.08 degrees and PL = 3.24 degrees. Therefore, at the bottom of the sinusoidal wave, the interval between the light-shielding portions 6a is 0.16 degrees. This corresponds to about 27.9 mu m when the radius of the optical scale 3 on the optical pattern 6 is 10 mm, for example. This is also true when the light is modulated only by the width Wn of the light-shielding portion 6a. That is, when the above value is used, the likelihood of the interval at which the light-shielding portion 6a is densest can be approximately tripled by defining the equation (7) Thereby facilitating manufacture.

When the width W of the light-shielding portion 6a is constant and only the pitch Pn of the light-shielding portion 6a is modulated, the interval at which the light-shielding portion 6a becomes the closest is defined as 0.16 degrees PL is 1.16 degrees and the transmittance at the bottom of the sinusoidal wave is about 14%, and the amplitude is reduced to about 1/3 as compared with the case of the formula (7).

As described above, according to the present embodiment, the difference between the maximum light quantity and the minimum light quantity becomes larger, and the amplitude of the received light can be increased, so that the influence of the angular error due to the offset error can be reduced. As a result, the absolute rotation angle? Of the optical scale can be measured with high accuracy and high resolution. Further, since the interval at the portion where the light-shielding portion 6a is the closest can be made large, the optical scale 3 can be easily manufactured.

(Example 2)

Fig. 8 is an explanatory diagram showing Embodiment 2 of the present invention. Fig. The optical encoder of this embodiment has the same configuration as that of the optical encoder 1 of the embodiment 1 except that the width W'n of the light shield 6a of the optical pattern 6 is expressed by the following equation (17) and (17a). However, the coefficient m is a real number greater than zero.

Other configurations and detection principles are the same as those of the first embodiment, and therefore only the different constituent elements will be described below, and a description of the same portions will be omitted.

Hereinafter, a case where a resin material is used as the base material of the optical scale 3 and a projection having a V-shaped cross section is formed as the light shielding portion 6a is exemplified. However, the periodic structure of the transmission portion and the light shielding portion, And is not particularly limited as long as it is an optical scale 3 having a periodic structure.

when the pitch Pn of the light-shielding portion 6a is large, the width W'n of the light-shielding portion 6a is defined as the width Wn of the light-shielding portion 6a by defining the relationship between the pitch Pn and the width W'n of the n- When the pitch Pn of the light-shielding portion 6a is small, the width W'n of the light-shielding portion 6a can be made large. As a result, the difference between the maximum light quantity and the minimum light quantity becomes larger, and the light intensity with a larger amplitude can be obtained in the light receiving section 4. [

When W'n in equation (17) is substituted into Wn in equation (12), the pitch Pn of shielding section 6a is given by the following equation (18).

For example, assuming that the width WH of the light-shielding portion 6a at the peak position of the sine wave is 1 degree, the pitch PH of the light-shielding portion 6a at the peak position of the sine wave is 10 degrees, and the coefficient m is 2, , The integer A = 10 and the transmittance at the bottom position of the sinusoidal wave is TL = 5%, the pitch Pn of the shielding portion 6a with respect to the n-th angle and the angle? (Table 3).

Here, as in the first embodiment, it is necessary to correct the angle of the last light-shielding portion 6a to match the origin. Therefore, the angle? N 'corresponding to the position of the nth light-shielding portion 6a after correction is corrected according to the equation (14) using the angle? Of the last light-shielding portion 6a. The pitch Pn of the light-shielding portion 6a and the angle? N 'of the nth light-shielding portion 6a with respect to the n-th angle at this time are as shown in Table 4 below.

From the expressions (16) and (17), when the width W'L of the light-shielding portion 6a and the pitch PL of the light-shielding portion 6a in the portion where the sinusoidal wave is the closest are obtained, W'L = 2.12 degrees , PL = 2.23 deg., And the interval between the light-shielding portions 6a at the bottom of the sine wave is 0.11 degrees, which is slightly smaller than in the case of m = 1 described in the first embodiment. However, as shown in Table 3, since the number of gradations constituting the sinusoidal wave of one rotation cycle becomes large and the angle before the angle correction in Equation (14) almost coincides with the origin position, The error is reduced, and more accurate detection becomes possible.

As described above, according to the present embodiment, the difference between the maximum light quantity and the minimum light quantity becomes larger, and the amplitude of the received light can be increased, so that the influence of the angular error due to the offset error can be reduced. As a result, the absolute rotation angle? Of the optical scale can be measured with high accuracy and high resolution. Further, since the light-shielding portion 6a can take a large distance at the densest portion, the optical scale 3 can be easily manufactured. In addition, since the number of gradations constituting the sinusoidal wave of one rotation cycle is increased, the error from the ideal sinusoidal wave becomes small, and high-precision detection becomes possible.

1: optical encoder 2: transparent portion
3: Optical scale 4: Light receiving part
5: computing unit 6: optical pattern
6a: shielding part 6b:
41, 42: light receiving element T1, T2: track
S1, S3: Sine wave output S2, S4: Sine wave output
S1α: Sinusoidal output after fluctuation of light quantity

Claims (3)

An optical scale having a periodic optical pattern, a relatively angular displacement,
A transparent portion for irradiating light toward the optical scale,
A light receiving unit for receiving light from the optical scale,
Based on a signal from the light-receiving unit, an arithmetic unit
And,
The optical pattern includes a plurality of light-shielding portions and a plurality of light-transmitting portions alternately arranged,
The transmittance T (n) corresponding to the angle n of the n-th light-shielding portion and the pitch Pn (n) of the light-shielding portion corresponding to the angle n of the n-th light-shielding portion are denoted by Pn and Wn, respectively, And the width Wn of the light shielding portion of the optical pattern satisfies the following expressions (A1) and (A2) using the pitch Pn, integer A and coefficient m (m is a real number larger than 0) (A3). ≪ / RTI >
[Equation 1]

The method according to claim 1,
The optical transmittance T (? N) of the optical pattern is represented by the following equation, where TH is the maximum transmittance and TL is the minimum transmittance.
&Quot; (2) "

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